Aqueous polymer dispersion, method for making same, and use thereof for preparing paints

ABSTRACT

Aqueous dispersion of polymer particles comprising at least one first polymer and at least one second polymer which are mutually incompatible. The first polymer is non-crosslinked, has a glass transition temperature of from -25° to 0° C. and represents from 40 to 65% by weight of the particles; the second polymer is not alkali-soluble and is non-crosslinked, has a glass transition temperature of from 5° to 40° C. and represents from 60% to 35% by weight of the particles; the glass transition temperature of the second polymer exceeds that of the first polymer by less than 40° C. and the average particle size is between 100 and 300 nm. 
     Application to the production of environmentally compatible paints.

The present invention relates to an aqueous polymer dispersioncontaining structured particles, which is intended in particular for theproduction of solvent-less paints, to a process for its production, andto its application to solventless paint formulations.

Latex-based paint compositions containing opacifying pigments, such astitanium dioxide, and non-opacifying pigments have been known for a longtime. When an aqueous polymer emulsion is applied to a substrate, andafter the water has evaporated, a film appears on the surface of thesaid substrate owing to the coalescence of the polymer which is presentin the emulsion.

In order to obtain a homogeneous film, the forces of coalescence mustexceed the resistance of the polymer particles to deformation and toviscoelasticity. The viscoelastic nature of the polymer particles,consequently, is the major factor determining the tendency of the latexto form a film.

It is established art to use a plasticizer in order to modify theviscoelasticity of a polymer latex. The plasticizers may be solids witha low melting point (for example dibutyl or dioctyl phthalate) or elsesolvents having a high boiling point. In a general manner, they increasethe elongation and the elasticity but reduce the breaking strength ofthe resin in which they are incorporated. Despite being less volatilethan the solvents, they are nevertheless in possession of acharacteristic odour.

It is also known to use solvents (of the glycol type, for example),sometimes called coalescence agents, in order to modify the viscoelasticnature of the polymer. These are volatile plasticizers in that theyfacilitate the elastic deformation of the particles of the latex.

The result of using plasticizers and coalescence agents in latex filmsis a greater degree of coalescence within an extended temperature range,and up until now it has been thought that their presence was essentialto good film formation.

Moreover, it is known to classify paints according to their gloss,measuredly reflection of light at a given angle, for example inaccordance with the standard DIN 53778 (Part 1), into:

matt paints: reflection greater than 7% but less than 45% at an angle of85°.

semi-matt paints: reflection greater than 45% at an angle of 85°.

semi-gloss paints: reflection greater than 31% but less than 62% at anangle of 60°.

The properties expected of a paint are good applicability, good bindingpower, the absence of cracks even at low temperature (5° C.), theabsence of a tacky or sticky nature, good resistance to wet abrasion(washability) and, finally, a degree of reflection of light which issuited to one or other of the above categories. According to which oneof these categories it belongs to, the pigment volume concentration ofthe paint is very different: from 70% to 95%, approximately, for a mattpaint, and only from 20% to 40%, approximately, for a semi-gloss paint.Likewise, washability is a property which is easily obtained insemi-matt and semi-gloss paints because of the high proportion of latexin the latter. In contrast, in matt paints which usually contain onlyfrom 10% to 15%, approximately, by weight of latex polymer, satisfactorywashability can only be obtained by careful selection of the nature ofthe latex polymer.

Moreover, it is well known that the paint industry causes a certainnumber of problems with regard to ecology and to respect for theenvironment, which result from the chemical composition of the paintand, in particular, from the presence in this composition of volatileorganic substances such as the coalescence agents.

These problems are, in particular, those brought about by the toxicityof these organic substances when the paint is produced and applied, andby the destruction of the ozone layer of the atmosphere when thesesubstances evaporate during application.

Aqueous polymer dispersions comprising non-homogeneous or structuredparticles obtained, in general, by the technique of overpolymerization,carried out in at least two polymerization steps, are well known.Examples of such structured particles, of varied morphology, have beendescribed by J. C. Daniel in Macromol. Chem., Suppl 10/11, pages 359 to390 (1985) and are designated, in particular, using the termscore-shell, mutlilayer, cellular, Poow or else particles with acomposition gradient. Other examples of structured particles, having an"octopus ocellatus" [ocellated octopus] morphology, have been describedby M. Okubo, K. Kanaida and Matsumoto in Colloid and POlymer Science,Vol. 265 No. 10 (1987), pages 876 to 881.

Thus patent U.S. Pat. No. 4,107,120 describes an emulsion of acrylicpolymer particles which have a core-shell structure in which the core iscrosslinked, has a glass transition temperature of less than -20° C. andrepresents from 30% to 60%, approximately, by weight of the particle,and in which the shell, which is likewise crosslinked, has a glasstransition temperature of from -10° to +60° C., approximately. PatentEP-B-151,360 describes an aqueous dispersion of polymer particlesconsisting of a copolymer of (A) from 95% to 99% by weight of at leastone alkyl (meth)acrylate in which the alkyI group has 1 to 8 carbonatoms, and (B) from 1 to 5% by weight of at least one water-solublemonomer which is copolymerizable with the monomer(s) (A), the saidparticles having a composite structure comprising a core and a shell,and the dispersion being characterized in that the shell contains from50 to 90 mol% of the water-soluble monomer(s) of the dispersion and inthat the said dispersion can be obtained by a process comprising a firststep of emulsion polymerization, in water, of the monomer(s) (A) and asecond step consisting in introducing into the reactor, when the degreeof progress of the polymerization reaction has reached a value of atleast 82%, a mixture comprising from 12 to 45% by weight of at least onewater-soluble monomer (B) and from 55 to 88% by weight of monomers (A).

U.S. Pat. No. 5,185,387 describes an aqueous synthetic-resin dispersionwhich has a film formation temperature of less than 50° C. and containsan emulsion of a polymer of composite structure comprising a core and ashell consisting of:

A) From 65 to 90% by weight of a polymer (core) having a low degree ofcrosslinking and a glass transition temperature of less than 0° C. andan elongation break of at least 150%, the said core comprising:

1) at least one (mono) ethylenically unsaturated monomer which ispolymerizable by free radicals and whose homopolymer has a glasstransition temperature of less than 25° C., and

2) from 0.1 to 2.0% by weight of the said core, of at least onecrosslinking monomer containing at least two ethylenically unsaturated,non-conjugated groups which are polymerizable by free radicals, and

B) from 10 to 35% by weight of a polymer (shell) which is essentiallynon-crosslinked and has a glass transition temperature of less than 60°C., comprising:

1) at least one ethylenically unsaturated monomer which is polymerizableby free radicals and whose homopolymer has a glass transitiontemperature of less than 25° C., and

2) at least one monoethylenically unsaturated monomer which ispolymerizable by free radicals and whose homopolymer has a glasstransition temperature of greater than 25° C.,

the glass transition temperature of the core being less than that of theshell by at least 10° C., and the shell containing up to 20% by weightof monomers chosen from acrylic acid, methacrylic acid and hydroxyalkylesters thereof, acrylamide, methacrylamide and N-alkylated derivativesthereof.

Patent application EP-A-522,789 describes a polymer binder in emulsionwhich comprises at least one first polymer and at least one secondpolymer which are mutually incompatible, in which the first polymer hasa glass transition temperature of from -30° to -60° C., approximately,and represents from 70% to 95%, approximately, by weight of the binderand in which the second polymer has a glass transition temperature offrom 0 to 60° C., approximately. The glass transition temperature of thesecond polymer (shell) is preferably greater than that of the firstpolymer (core) by at least approximately 40° C.

U.S. Pat. No. 4,868,016 describes a composition which is formed bycombining at least one thermoplastic polymer latex which issubstantially insoluble in an aqueous-alkaline medium with at least onealkali-soluble polymer which is incompatible with the said thermoplasticpolymer latex, in a ratio by weight of the thermoplastic latex to thealkali-soluble polymer which is between approximately 20:80 andapproximately 99:1. The thermoplastic latex preferably possesses a glasstransition temperature of less than -10° C. and the alkali-solublepolymer possesses a glass transition temperature which is greater than30° C. The alkali-soluble polymer preferably comprises from 10% to 60%by weight of an acidic monomer (such as (meth)acrylic acid), while thethermoplastic latex preferably comprises less than 3% by weight of anacidic monomer of this kind. The composition described by this documentis useful as an adhesive, since it becomes sticky to the touch when itstemperature is raised.

The document EP-A-429,207 describes, primarily for the coating of wood,glass, plastics and aluminium, a composition which comprises an aqueousdispersion of particles comprising at least two mutually incompatiblecopolymers, forming an outer phase and an inner phase, whose averagesize is from 20 to 70 nm. Crosslinking monomers may be present. The Tgof the outer phase is preferably lower than that of the inner phase, theone being preferably lower than 35° C. and the other preferably greaterthan 45° C., approximately.

However, up to now there has been no aqueous polymer dispersion--ofwhich the documents above illustrate some examples--which has made itpossible to do without coalescence agents in such a way as to solve theenvironmental problems described previously.

The aim of the present invention is to attain a number of objectives,and in particular to solve the ecological problems described above bytotally eliminating the coalescence agents from the composition of theaqueous paints. Secondly, it alms to attain the first objective withoutdetriment to the totality of properties demanded of the paint. It alsoaims to define a polymer binder which is adapted to matt, satin andsemi-gloss paints, whilst obtaining, especially for matt paints (whichcontain little binder) an improved degree of washability.

In order to attain these various objectives, a first subject of thepresent invention consists of an aqueous dispersion of polymer particlescomprising at least one first polymer and at least one second polymerwhich are mutually incompatible, characterized in that the first polymeris non-crosslinked, has a glass transition temperature of from -25° to0° C., approximately, and represents from 40 to 65%, approximately, byweight of the particles, the second polymer is not alkali-soluble and isnon-crosslinked, has a glass transition temperature of from +5° to 40°C., approximately, and represents from 60% to 35%, approximately, byweight of the particles, in that the glass transition temperature of thesecond polymer exceeds that of the first polymer by less than 40° C.,and in that the average particle size is between 100 and 300 nm.

The two mutually incompatible polymers making up the aqueous dispersionaccording to the invention may consist of any morphologicalconfiguration of structured particles, such as those described by J. C.Daniel in Macromol. Chem., Suppl. 10/11, pages 359 to 390 (1985) and byM. Okubo, K. Kanaida and T. Matsumoto in Colloid and Polymer Science,Vol. 265 No. 10 (1987), pages 876 to 881, namely:

core-shell.

core-shell with multiple cores.

core-shell where the shell surrounds the core incompletely.

"octopus ocellatus".

The mutual incompatibility of the two polymers of the dispersion may bedetermined by any method which is known in the art, in particular byelectron microscopy when the two polymers are of different types.

With regard to the second polymer, the term not alkali-soluble isunderstood as meaning insoluble in an aqueous-alkaline solution, whichimplies that the proportion of carboxylic acid monomer in this polymerdoes not exceed approximately 8% by weight.

The glass transition temperatures of the two polymers of the dispersion,which constitute characteristics which are very important for theefficacy of the present invention, may be calculated using the Foxequation (T. G. Fox, Bull. Am. Physics Soc., Vol. 1 (3), page 123(1956)) and are in general measured by the method of programmeddifferential calorimetry.

Each of the two mutually incompatible polymers making up the aqueousdispersion according to the invention is prepared by polymerization ofat least one ethylenically unsaturated monomer, observing a monomercomposition which makes it possible to attain the desired glasstransition temperature. Among the ethylenically unsaturated monomerswhich can be used it is possible to mention, on the one hand, esters ofacrylic acid or methacrylic acid, such as methyl acrylate, ethylacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylateand n-butyl methacrylate, and, on the other hand, aromatic vinylmonomers such as styrene and its derivatives, for example alpha-methylstyrene, vinyl toluene and tert-butyl styrene. The monomers forming thefirst polymer may be identical to or different from those forming thesecond polymer. Particular preference is given to using:

in order to form the core of the structured particle (first polymer), amixture of from 15 to 35%, approximately, by weight of styrene or ofmethyl methacrylate and from 65 to 85%, approximately, by weight ofn-butyl acrylate.

in order to form the shell of the structured particle (second polymer),a mixture of from 40 to 70%, approximately, by weight of styrene andfrom 30 to 60%, approximately, by weight of n-butyl acrylate.

Each polymer making up the aqueous dispersion according to the inventionmay additionally comprise, as an option, up to approximately 2 parts byweight of at least one water-soluble comonomer which is copolymerizablewith the ethylenically unsaturated monomers, per 100 parts by weight ofthe said monomers. As water-soluble comonomers which can be used it ispossible to mention, in particular, acrylic acid, methacrylic acid,acrylamide and methacrylamide. However, in contrast to the teaching ofPatent EP-B-151,360, there is no need for this water-soluble comonomerto be incorporated selectively into the shell of the structuredparticle. It may simply be present in the core and in the shell insubstantially identical proportions.

According to preferred embodiments of the present invention:

the glass transition temperature of the first polymer is between -25°and -5° C., approximately.

the glass transition temperature of the second polymer is between +15°and +35° C., approximately.

the glass transition temperature of the second polymer exceeds that ofthe first polymer by more than 20° C.

The methods of measuring and controlling the average particle size arewell known to the person skilled in the art, for example by way of E. A.Collins, 18th Annual Short Course (June 1987) of the Institute ofPolymer Emulsion, Lehigh University (Pennsylvania), by way of E. A.Collins, J. A. Davidson and C. A. Daniels, J. Paint Technology 47, 35(1975) and via the operating principles of the AutoSizer Lo-C apparatusfrom Malvern Instruments.

As already set out in the general exposition of the present invention,it is essential for the two mutually incompatible polymers of thedispersion to be non-crosslinked. For this reason, no polyethylenicallyunsaturated monomer should be present in the compositions of monomersused to synthesize these polymers.

A second subject of the present invention consists of a process for theproduction of an aqueous dispersion, such as that according to theinvention, comprising a first step of emulsion polymerization ofmonomers, leading to the formation of a first polymer, followed by asecond step of emulsion polymerization of monomers, leading to theformation of a second polymer which is incompatible with the first,characterized in that:

the monomers used in the first step lead to the formation of a first,non-crosslinked polymer which has a glass transition temperature of from-25° C. to 0° C. , approximately,

the monomers used in the second step lead to the formation of a second,non-crosslinked polymer which is not alkali-soluble, having a glasstransition temperature which is from +15° C. to 40° C., approximately,and exceeds the glass transition temperature of the first polymer byless than 40° C.,

the quantities of monomers employed in the first step and in the secondstep are such that the first polymer represents from 30% to 65%,approximately, by weight of the particles of dispersion, while thesecond polymer represents from 70% to 35%, approximately, by weight ofthe particles of the dispersion, and

the emulsion polymerization conditions in the first step and in thesecond step are such that the average particle size is between 100 and300 nm.

In order to lead to the formation of a first, non-crosslinked polymerand a second, non-crosslinked polymer, the flows of monomers used in thefirst step and, respectively, in the second step of the processaccording to the invention do not contain any monomer which iscrosslinking as commonly designated in this art, that is to say nomonomer which comprises at least two ethylenically unsaturated groupswhich are polymerizable by free radicals.

For the preparation of an aqueous dispersion which is suitable for theformulation of matt paints, as defined above, it is preferablefurthermore that the addition and polymerization of the monomers in thesecond step commence when the composition of monomers of the first stephas a degree of conversion of no more than 90%.

The nature of the monomers used in the first step and in the second stepof the process according to the invention has already been set outabove. As already indicated, it is possible to employ, in the first stepand in the second step of the process, up to approximately two parts byweight of a water-soluble comonomer which is copolymerizable with theprincipal monomers per 100 parts by weight of the said monomers.

The process according to the invention can be carried out in thepresence of up to approximately 1 part by weight, per 100 parts byweight of the monomers, of at least one chain transfer agent, in orderto regulate the number-average molecular weight of the resultingpolymer. As examples of compounds which can be used as chain transferagents in the present invention, it is possible to mentionmercaptocarboxylic acids having from 2 to 8 carbon atoms and theiresters, such as mercaptoacetic acid, 2-mercaptopropionic and3-mercaptopropionic acids, 2-mercaptobenzoic acid, mercaptosuccinicacid, mercaptoisophthalic acid and alkyl esters thereof. Preference willbe given to using a mercaptomonocarboxylic acid and/or amercaptodicarboxylic acid having from 2 to 6 carbon atoms, moreparticularly a mercaptopropionic acid and its alkyl esters, specificallythe isooctyl or n-dodecyl esters of mercaptopropionic acid.

The principal flow of the ethylenically unsaturated monomersconstituting each polymer of the dispersion may be emulsified by meansof a least one anionic or nonionic surfactant, or may simply beintroduced into the reactor in the form of a homogeneous mixture ofmonomers. In the latter case, an aqueous solution of one or moresurfactants may be added simultaneously. It is preferable to use acombination of nonionic surfactant and anionic surfactant in order toprepare emulsions. As examples of nonionic surfactants it is possible tomention polyethers such as condensates of ethylene oxide and propyleneoxide, alkyl and alkylaryl ethers and thioethers of polyethylene glycolsand polypropylene glycols, alkylphenoxypoly(ethylenoxy)ethanols,polyoxyalkylene derivatives of partial esters of long-chain fatty acidssuch as lauric, myristic, palmitic and oleic acids, condensates ofethylene oxide with higher alkanethiols, ethylene oxide derivatives oflong-chain carboxylic acids and of alcohols, etc. These nonionicsurfactants preferably contain from approximately 5 to 100 units ofethylene oxide per molecule and, even more preferably, fromapproximately 20 to 50 of such units. As anionic surfactants which canbe used, preferably in combination with the said nonionic surfactants,it is possible to mention sulphates and sulphonates of high molecularweight, for example alkyl, aryl and alkylaryl sulphates and alkyl-,aryl- and alkylarylsulphonates of sodium and potassium, such as sodium2-ethylhexyl sulphate, potassium 2-ethylhexyl sulphate, sodium nonylsulphate, sodium undecyl sulphate, sodium tridecyl sulphate, sodiumpentadecyl sulphate, sodium lauryl sulphate, sodiummethylbenzenesulphonate, potassium methylbenzenesulphonate, potassiumtoluenesulphonate and sodium xylenesulphonate, the sulphonatedderivatives of the nonionic surfactants listed above, dialkyl esters ofalkali metal salts of sulphosuccinic acid, such as sodiumdiamylsulphosuccinate, and condensation products offormaldehyde/naphthalenesulphonic acid. The total quantity ofsurfactants used in the emulsion polymerization process of the presentinvention varies from approximately 2 to 20 by weight, preferablyapproximately from 4 to 12% by weight, of the monomeric components. Theratio by weight of the anionic surfactant to the nonionic surfactantshould be between 0.01 and 1 approximately, preferably betweenapproximately 0.05 and 0.5. The quantity of water used in the reactionmedium is, in general, determined by the solids content desired in theaqueous dispersion according to the invention, which is generallybetween approximately 40% and 70%, preferably between 45 and 60% byweight.

The monomeric components of the dispersion according to the inventionare polymerized by means of effective quantities, preferably between 0.1and 2%, approximately, by weight of the total charge of monomers, of atleast one conventional free-radical initiator. Such an initiator ispreferably substantially soluble in water. Such initiators compriseinorganic peroxides such as hydrogen peroxide, alkali metal perboratesand persulphates, and redox systems such as combinations of an ammoniumperchlorate, perborate or persulphate or of an alkali metal with analkali metal bisulphite.

The polymerization temperature required to produce the aqueous polymersof the present invention in each of the two steps of the process isgenerally within a range from approximately 40° to 95° C. --preferablyfrom approximately 55° to 85° C. --depending on the time envisaged forthe polymerization. The polymerization time is generally fromapproximately 45 minutes to 6 hours for each of the two steps, this timebecoming higher as the polymerization temperature decreases.

In order to attain a final degree of conversion in the polymerizationreaction of 100%, it may be desirable to follow the second step bycooking of the aqueous polymer emulsion for approximately 30 to 90minutes at a temperature which is higher, preferably by at least 8° C.,than the polymerization temperature.

A complementary improvement of the present invention consists in thetreatment of the aqueous polymer emulsion, after the second step or, ifappropriate, after the cooking step, by means of a free-radicalinitiation system which has a short half-life at the temperature underconsideration, in order to attain an overall degree of conversion whichis close to 100% and/or a residual monomer content which does not exceedapproximately 50 ppm. As examples of free-radical initiator systems itis possible to mention organic and inorganic peroxides such astert-butyl hydroperoxide, butyl peroxide, hydrogen peroxide or alkalimetal persulphates, in combination with a reducing agent such as sodiumformaldehyde-sulphoxylate, ascorbic acid, Mohr's salt, etc. Such atreatment may be carried out at temperatures from 40° C. to 90° C.,approximately, its duration depending on the chosen temperature andbeing preferably between 15 minutes and 3 hours, approximately.

When the latex prepared in conformity with the present invention appearstoo acid to enter into the formulation of paints, it may be desirable toadjust its pH to a value of greater than 7, for example by means of anyalkaline substance such as the hydroxides of sodium, potassium orammonium.

The first step of the process according to the invention may be seeded,that is to say initiated in the presence of a preformed emulsion ofparticles of small size, or else unseeded. Seeded polymerization ispreferred and makes it possible to obtain polymer dispersions havingparticle sizes which are more uniform than with unseeded polymerization.More particularly, it has been discovered in accordance with the presentinvention that a very significant improvement in the washability of mattpaint, as described above, may be obtained by carrying out seeding inthe first step of the process such that approximately from 80 to 98% byweight of the monomers used in the first step are added to a reactionmedium which already contains a partially polymerized pre-emulsion ofapproximately 2 to 20% by weight of the said monomers, and in that theaverage size of the polymer particles present in the said partiallypolymerized pre-emulsion does not exceed approximately 95 nanometers(nm), and preferably 65 nm. In addition to the methods of particle-sizemeasurement already described previously, the person skilled in the artmay also check that the average size of the polymer particles present inthe partially polymerized pre-emulsion does not exceed 95 nm either byway of the polymerization time of the said partial flux (approximatelyfrom 5 to 20% by weight of the totality of the monomeric components) ofthe pre-emulsion, which should preferably not exceed approximately 45minutes, or else by the overall degree of conversion of the monomers inthe said partial flux of the pre-emulsion, which should reach at least50% and preferably at least 90% by weight.

Another subject of the present invention consists of a paint which isfree of plasticizer and of coalescence agent, comprising an aqueouspolymer emulsion of the type described above.

The formulation method employed may be any one of those which arecurrently known in the art of formulating latex paints. Essentially, theaqueous paints according to the invention comprise a mixture ofpigmented material and latex. The pigmentary material is preferably usedin a form which has been suitably reduced to a powder by conventionalmeans, and all of the other optional, solid adjuvants described below.

To combine the paint ingredients so as to formulate the latex paint ofthe present invention, one or more primary pigments and, if appropriate,other adjuvants (which may themselves be liquid or solid) may beincorporated into the latex binder in order to form a uniform mixture,simply by adding all of the raw materials to a stirred vessel. Althoughit is possible to combine the pigments and other ingredients with theemulsion and to grind or crush the mixture in a grinder-disperser, it isdesirable first to prepare an aqueous paste of the pigment andadjuvants, preferably with the aid of a dispersant, in an apparatushaving a high shear gradient, and then to combine the resulting pastewith the latex. The consistency of the paste may be controlled by therespective quantities of water, pigments and adjuvants which are used.

To formulate the aqueous paints of the present invention it is possibleto use:

filler materials which are present in pulverulent form, such as calciumcarbonate, dolomite, talc, mica, barium sulphate, lime, cement, and anyfiller materials conventionally used in paints.

pigmentary materials such as titanium oxide, carbon black, copperphthalate cyanine, zinc oxide, iron oxides, chromium oxide and any otherpigmentary material which is conventionally employed in paints.

The proportion of pigmentary material which is used to formulate anaqueous paint according to the present invention is determined by acertain number of factors including the depth of colour desired, thehiding power, etc. In general, the aqueous paints of the presentinvention possess pigment volume concentrations within a range fromapproximately 20% to 95% according to the category of paint in question;for example, from approximately 70 to 95% for matt paints and fromapproximately 20 to 40% for semi-gloss paints.

In addition to the pigments and fillers, one or more adjuvants may ifdesired be included in the formulation of the aqueous paints of thepresent invention. Such ingredients include (but are not limited to):

dispersants, also called deflocculants, that is to say compounds whichare capable of promoting the formation of electrical charges of the samesign at the surface of the polymer particles and, consequently, ofpromoting the creation of repellent electrical forces between theseparticles, such as alkali metal silicates (especially metasilicates),alkali metal polyphosphates and alkali metal salts of organic polyacids(especially polyacrylates);

wetting agents, that is to say compounds capable of modifying thesurface tension of the medium, generally comprising groups ofhydrophilic and hydrophobic nature. They may include anionic surfactants(for example alkali metal alkylarylsulphonates), cationic surfactants(for example quaternary ammonium salts) or nonionic surfactants (forexample polyether oxides);

rheology modifiers or thickeners, such as water-soluble polymersmodified by hydrophobic groups (for example urethane ethoxylate modifiedhydrophobically) and hydroxyalkylcellulose derivatives;

inorganic bases, that is to say, for example, ammonia, sodium hydroxideor potassium hydroxide;

antifoam agents such as a mixture of mineral oil and nonionicemulsifier;

biocides, that is to say compounds which are capable of destroyingmicroorganisms, such as isothiazolone or chloroallyl derivatives;

anticorrosive agents, such as an aqueous solution of sodium benzoateand/or of sodium nitrite;

insecticides, fungicides, acaricides and other pesticides, such aspyrethrinoid compounds (for example decamethrin or dieldrin), lindane,N-heterocyclic compounds, uracyclic compounds, triazole derivatives,organophosphorus compounds, etc.

The choice of the particular adjuvants and their respective quantitiesis generally dictated by the properties desired for a particular aqueouspaint, and is within the scope of the person skilled in the art. Ingeneral, however, the latex paints of the present invention have anoverall solids content of between approximately 40 and 70% by weight.

The aqueous paints of the present invention may be applied to thesurface of a wall or to any other substrate by any conventional means,for example brush, roller, spray-gun, etc. The paint may be crosslinkedor dried after application to the substrate, likewise by conventionalmeans, for example drying in air, baking, etc. The coat of paint,crosslinked or otherwise, has excellent performance characteristics asregards resistance to yellowing, absence of cracking at low temperatureand of any tacky or sticky nature, and coalescence at low temperature.Moreover, the surfaces coated using the latex paints according to thepresent invention have good characteristics with regard to the contrastratio (that is to say the hiding power).

These properties are illustrated more fully in the examples whichfollow, which are given with the aim of illustrating--but notlimiting--the present invention. All of the quantities without exceptionare expressed in terms of weight.

EXAMPLES 1 to 6

A first emulsion is prepared at room temperature (23° C.) by adding thefollowing ingredients to a vessel equipped with a stirrer, in thefollowing order:

    ______________________________________                                        Demineralized water       32 parts                                            Anionic surfactant marketed by                                                                          0.75 part.sup.                                      Schering under the tradename Rewopol                                          NOS 25 (35% active ingredient)                                                Nonionic surfactant marketed by                                                                         3.6 parts                                           Schering under the tradename Rewopol                                          HV 25 (80% active ingredient)                                                 Acrylamide                0.2 part.sup.                                       Acrylic acid              1 part.sup.                                         n-Butyl acrylate          x parts                                             Styrene                   y parts                                             n-Dodecanethiol           0.02 part.sup.                                      ______________________________________                                    

The reactor is charged with 5% by weight of the first emulsion, 38 partsof demineralized water and 0.3 part of Rewopol NOS 25. The reactor isthen heated to 72° C. and then maintained at this temperature. Asolution of metabisulphite (0.1 part of sodium metabisulphite in 0.5part of demineralized water) and an initiator solution (0.035 part ofammonium persulphate in 0.5 part of deminizeralized water) are thenadded to the reactor. Polymerization is then carried out for 15 minutes.The reactor is maintained at a temperature of 72° C. and

the remaining 95% by weight of the first emulsion and

an initiator solution (0.315 part of ammonium persulphate in 10 parts ofdemineralized water) are added to the seed batch thus obtained,simultaneously and progressively over an additional period of 2 hours.

During this first polymerization step, a second emulsion is alsoprepared at room temperature (23° C.) by adding the followingingredients to a vessel equipped with a stirrer, in the following order:

    ______________________________________                                        Demineralized water    20 parts                                               Rewopol HV 25          2.8 parts                                              Acrylamide             0.3 part.sup.                                          Acrylic acid           1.5 part.sup.                                          n-Butyl acrylate       w parts                                                Styrene                z parts                                                n-Dodecanethiol        0.1 part.sup.                                          ______________________________________                                    

At the end of the first step of polymerization, the second emulsion--onthe one hand--and, on the other hand, an initiator solution (0.315 partof ammonium persulphate in 10 parts of demineralized water) are added tothe reactor. This addition lasts 90 minutes, after which the temperatureis maintained at 72° C. for 15 minutes more, and then cooking is carriedout at 80° C. for 1 hour. The latex is subsequently cooled to 60° C. andthen treated by adding to it:

firstly, a solution of 0.315 part of hydrogen peroxide (100% activeingredient) in 2.3 parts of demineralized water,

then a solution of 0.015 part of Mohr's salt and 0.315 part of ascorbicacid in 1.7 parts of demineralized water, which is added over the courseof an hour.

The latex is then cooled to 30° C. before a 10% aqueous solution ofsodium hydroxide is added to it over the course of an hour.

By varying, on the one hand, the values of x, y, w and z and, on theother hand, the degree of conversion τ (expressed as % by weight) of themonomers at the end of the first step, different latexes are obtained,for which the following characteristics are indicated in Table I:

solids content SC, expressed as a percentage.

average particle size APS, determined using an AutoSizer Lo-C apparatusfrom Malvern Instruments and expressed in nanometers.

Brookfield viscosity BV at 100 revolutions/minute, expressed in Pa.s.

residual monomer content RMC, expressed in ppm.

minimum film formation temperature MFT, expressed in ° C.

pH.

glass transition temperatures of the core, Tg1, and of the shell, Tg2,expressed in ° C. and determined by programmed differential calorimetry.

                  TABLE I                                                         ______________________________________                                        Example                                                                              1       2        3     4     5      6                                  ______________________________________                                        τ  83      83       83    89    95     99                                 x      25.8    37       40.8  37    37     37                                 y      8.2     11.8     13    11.8  11.8   11.8                               w      21      16.7     15    16.7  16.7   16.7                               z      42      31.5     28.2  31.5  31.5   31.5                               SC     49.1    48.7     48.8  48    48.2   48.9                               APS    122     116      125   116   117    120                                BV     0.25    0.3      0.35  0.27  0.3    0.31                               RMC    30      20       26    20    32     26                                 MFT    0       0        0     0     0      0                                  pH     8.5     8.7      8.5   8.5   8.7    8.5                                Tg1    -12     -12      -12   -11   -12    -12                                Tg2    22      22       21    24    28     33                                 ______________________________________                                    

Example 1 is comparative

EXAMPLES 7 to 12

Some aqueous polymer dispersions from the preceding examples wereformulated into matt paint compositions, comprising:

    ______________________________________                                        water                     270 parts                                           wetting agent marketed by Benkiser                                                                      3 parts                                             under the tradename Lopon 890                                                 biocide marketed Riedel-DeHaen                                                                          1 part.sup.                                         under the tradename Mergal K 10                                               wetting agent and dispersant marketed                                                                   2 parts                                             by Munzing under the tradename Agitan 281                                     thickener (rheology modifier) marketed by                                                               4.5 parts                                           Hoechst under the tradename MHB 6000 Y                                        titanium oxide marketed by Sachtleben                                                                   50 parts                                            under the tradename Hombitan R 611                                            calcium carbonate marketed by Deutsche                                                                  160 parts                                           Solvay under the tradename Socal P 2                                          chalk (industrial grade)  250 parts                                           calcium carbonate marketed by Omya                                                                      110 parts                                           under the tradename Calcidar BL                                               silica marketed by Langer under the                                                                     10 parts                                            tradename Dicalita WP 5                                                       25% aqueous sodium hydroxide solution                                                                   2 parts                                             aqueous polymer emulsion  117.6 parts                                         ______________________________________                                    

The washability of these paints was measured after 8 days in accordancewith the standard DIN 53778 Part 2. Table II indicates the values forwashability, expressed in cycles, as a function of the nature of theaqueous polymer emulsion employed.

                  TABLE II                                                        ______________________________________                                        Example    7       8       9    10    11   12                                 ______________________________________                                        Aqueous polymer                                                                          1       2       3    4     5    6                                  from Example                                                                  W          200     800     760  510   300  260                                ______________________________________                                    

Examples 7, 11 and 12, of mediocre washability, are given by way ofcomparison.

Moreover, the aptitude of these paints in the formation of a film at 5°C. was evaluated in the following way: the paint formulation is appliedwhile still in the wet state to a glass plate in a thickness of 300 μm.The plate is then stored at 5° C. at a relative humidity of 70% for 18hours, so that the formation of the film of paint takes place underextreme service conditions. One hour after return to room temperature(20° C.), visual observation is carried out of the presence or otherwiseof cracks.

The paints of Examples 8 to 10 have a good aptitude for film formationat 5° C., as characterized by the absence of cracks.

EXAMPLES 13 to 15

The aqueous polymer dispersions of Examples 1 to 6 were formulated intosemi-matt paint compositions, comprising:

    ______________________________________                                        water                     150 parts                                           dispersant marketed by Hoechst under                                                                    3 parts                                             the tradename Dispex N 40                                                     wetting agent and dispersant marketed                                                                   3 parts                                             by Munzing under the tradename Agitan 281                                     biocide marketed by Riedel-De Haen                                                                      1 part.sup.                                         under the tradename Mergal K 10                                               thickener marketed by Aqualon under                                                                     3 parts                                             the tradename Culminal HEC 30000                                              titanium dioxide marketed by Kronos                                                                     200 parts                                           under the name Kronos 2063                                                    calcium carbonate marketed by Omya                                                                      175 parts                                           under the tradename Hydrocarb                                                 neutralizing agent (25% aqueous                                                                         1 part.sup.                                         sodium hydroxide solution)                                                    aqueous polymer emulsion  350 parts                                           ______________________________________                                    

The reflection of light for these paints is determined in accordancewith the standard DIN 53778 (Part 1) at two different angles: 20° (R₂₀)and 60° (R₆₀). The corresponding values, expressed as a percentage, areindicated in Table III as a function of the nature of the aqueouspolymer emulsion employed.

                  TABLE III                                                       ______________________________________                                        Example           13         14    15                                         ______________________________________                                        Aqueous polymer emulsion                                                                        2          3     4                                          of Example                                                                    R.sub.20          1.4        1.0   2.0                                        R.sub.60          8.8        9.2   9.4                                        ______________________________________                                    

In addition, the aptitude of these paints for film formation at 5° C.was evaluated in accordance with the same method as for Examples 7 to12. This aptitude is good, as characterized by the absence of cracks.

Finally, the superficial tack is evaluated by the separation capacity oftwo films of paint in the wet state (of format 25×25 mm and of thickness300 μm) placed in contact, after drying for 24 hours at 23° C. at arelative humidity of 50%, for 8 hours under a load of 1 kg.

For each of Examples 13 to 15, a complete absence of blocking betweenthe two test specimens is observed.

EXAMPLE 16

The aqueous polymer dispersion of Example 2 is formulated into asemi-gloss paint composition, comprising:

    ______________________________________                                        water                     150 parts                                           wetting agent and dispersant marketed                                                                   2 parts                                             by Coatex under the name P 90                                                 thickener marketed by Aqualon under                                                                     5 parts                                             the name Natrosol 250 HR                                                      20% aqueous solution of sodium benzoate                                                                 10 parts                                            25% aqueous solution of sodium                                                                          2 parts                                             hydroxide as neutralizing agent                                               titanium dioxide marketed by Tioxide                                                                    250 parts                                           under the name TR 92                                                          thickener marketed by Coatex under                                                                      16 parts                                            the name Coapur 5035                                                          aqueous polymer emulsion  540 parts                                           ______________________________________                                    

The aptitude of this paint for film formation at 5° C., and its surfacetack, were evaluated in accordance with the methods described in thepreceding examples and were found to be satisfactory, namely:

absence of cracking on film formation at 5° C.

total absence of blocking between the two test specimens.

The reflection of light for this paint is determined in accordance withthe standard DIN 53778 (Part 1) and is found to be:

44.1% at an angle of 60°.

85% at an angle of.85°.

EXAMPLES 17 and 18

Emulsions are prepared in accordance with the process described forExamples 1 to 6, except for the fact that the styrene is replaced bymethyl methacrylate in both the first step and the second step. Example18 is comparative. Depending on the values of x, y, w, z and τ, latexesare obtained for which the characteristics, expressed and determined asfor Examples 1 to 6, are indicated in Table IV below.

                  TABLE IV                                                        ______________________________________                                        Example         17      18                                                    ______________________________________                                        τ           82      88                                                    x               37      16.7                                                  y               11.8    31.5                                                  w               16.7    37                                                    z               31.7    11.8                                                  SC              48.9    49.1                                                  APS             152     142                                                   BV              2.0     1.9                                                   RMC             45      30                                                    MFT             0       0                                                     pH              8.5     8.5                                                   Tg1             -12     +38                                                   Tg2             +26     -10                                                   ______________________________________                                    

EXAMPLES 19 and 20

The aqueous polymer (latex) dispersions of Examples 17 and 18 areformulated into matt paint compositions which are strictly identical tothose of Examples 7 to 12, and for which the washability W is measuredunder the conditions already described. The values of W are indicated inTable V below. Example 20 is comparative.

                  TABLE V                                                         ______________________________________                                        Example             19     20                                                 ______________________________________                                        Aqueous polymer     17     18                                                 emulsion of Example                                                           W                   500    300                                                ______________________________________                                    

I claim:
 1. An aqueous dispersion of polymer particles comprising atleast one first polymer and at least one second polymer which aremutually incompatible, wherein the first at least one second polymerwhich are mutually incompatible, wherein the first polymer isnon-crosslinked, has a glass transition temperature of from -25° to 0°C. and represents from 40 to 65% by weight of the particles, the secondpolymer is not alkali-soluble and is non-crosslinked, has a glasstransition temperature of from 5° to 40° C. and represents from 60% to35% by weight of the particles, the glass transition temperature of thesecond polymer exceeds that of the first polymer by less than 40° C.,and the average particle size is between 100 and 300 nm.
 2. The aqueousdispersion according to claim 1, wherein the first polymer and thesecond polymer are prepared from at least one ethylenically unsaturatedmonomer which is chosen from among the esters of acrylic acid ormethacrylic acid and aromatic vinyl monomers.
 3. The aqueous dispersionaccording to claim 2, wherein the first polymer is formed from a mixtureof from 15 to 35% by weight of styrene or methyl methacrylate and from65 to 85% by weight of n-butyl acrylate, and the second polymer isformed from a mixture of from 40 to 70% by weight of styrene or methylmethacrylate and from 30 to 60% by weight of n-butyl acrylate.
 4. Theaqueous dispersion according to claim 2 wherein the first polymer or thesecond polymer additionally comprises up to 2 parts by weight of atleast one water-soluble comonomer which is copolymerizable with theethylenically unsaturated monomers, per 100 parts by weight of the saidmonomer.
 5. The aqueous dispersion according to claim 4, wherein saidwater-soluble comonomer is chosen from acrylic acid, methacrylic acid,acrylamide and methacrylamide.
 6. The aqueous dispersion according toclaim 1, wherein the glass transition temperature of the second polymerexceeds that of the first polymer by more than 20° C.
 7. The aqueousdispersion according to claim 5, wherein the glass transitiontemperature of the second polymer exceeds that of the first polymer bymore than 20° C.
 8. A process for the production of an aqueousdispersion comprising a first step of emulsion polymerization ofmonomers leading to the formation of a first polymer, followed by asecond step of emulsion polymerization of monomers, leading to theformation of a second polymer which is incompatible with the first,wherein:the monomers used in the first step lead to the formation of afirst, non-crosslinked polymer which has a glass transition temperatureof from -25° C. to 0°, the monomers used in the second step lead to theformation of a second, non-crosslinked polymer which is notalkali-soluble, having a glass transition temperature which is from 5°C. to 40° C. and exceeds the glass transition temperature of the firstpolymer by less than 40° C., the quantities of monomers employed in thefirst step and in the second step are such that the first polymerrepresents from 40% to 65% by weight of the particles of dispersion,while the second polymer represents from 60% to 35% by weight of theparticles of the dispersion, and the emulsion polymerization conditionsin the first step and in the second step are such that the averageparticle size is between 100 and 300 nm.
 9. The process according toclaim 8, wherein the addition and the polymerization of the monomers inthe second step commence when the composition of monomers of the firststep is at a degree of conversion of no more than 90%.
 10. The processaccording to claim 8, wherein the polymerization is carried out in thepresence of up to 1 part by weight, per 100 parts by weight of themonomers, of at least one chain transfer agent.
 11. The processaccording to claim 8, wherein in the first step seeding is carried outsuch that from 80 to 98% by weight of the monomers used in the firststep are added to a reaction medium which already contains a partiallypolymerized pre-emulsion of from 2 to 20% by weight of the saidmonomers, and the average size of the polymer particles present in thesaid partially polymerized preemulsion does not exceed 95 nanometers.12. The process according to claim 11, wherein the polymerization timeof the flux of partial preemulsion does not exceed 45 minutes.
 13. Theprocess according to claim 8, wherein the dispersion is prepared in thepresence of a combination of a nonionic surfactant and an anionicsurfactant.
 14. The process according to claim 8, wherein after thesecond step a treatment is carried out by means of a free-radicalinitiation system having a short half-life in order to attain a residualmonomer content which does not exceed 50 ppm.
 15. The process accordingto claim 11, wherein after the second step a treatment is carried out bymeans of a free-radical initiation system having a short half-life inorder to attain a residual monomer content which does not exceed 50 ppm.16. A paint composition which is free of plasticizer and coalescenceagent comprising an aqueous polymer dispersion according to claim
 1. 17.A paint composition which is free of plasticizer and coalescence agentcomprising an aqueous polymer dispersion according to claim
 3. 18. Apaint composition which is free of plasticizer and coalescence agentcomprising an aqueous polymer dispersion according to claim
 7. 19. Apaint composition which is free of plasticizer and coalescence agentobtainable by the process according to claim
 8. 20. A paint compositionwhich is free of plasticizer and coalescence agent obtainable by theprocess according to claim 11.